DE102008033828B4 - Electrochemical gas sensor - Google Patents

Abstract

Electrochemical gas sensor for detecting H2S in a gas sample with at least one measuring electrode (3) and a further electrode (8) in an electrolyte solution (9), wherein the electrolyte solution (9) contains a mediator compound in the form of transition metal metals, with a concentration of Metallates in a range between 0.2 molar and 2.0 molar and ionic liquids in the form of substituted ammonium, phosphonium or imidazolium compounds.

Description

The invention relates to an electrochemical gas sensor with a Mediatorverbindung.

An electrochemical gas sensor with a dissolved in the electrolyte mediator is from the DE 10 2004 062 051 A1 known. The presence of a mediator provides the ability to provide highly selective sensors to the analyte gas. The operation of a gas sensor with a mediator is based on the fact that the analyte gas is diffused by the measuring electrode into the electrolyte solution and oxidized or reduced by the mediator. In this case, the analyte is converted into a decomposition product and the mediator is converted into an intermediate product, which is reoxidized or redissolved at the measuring electrode. The required electron transfer, which is proportional to the proportion of the analyte gas in the gas sample, can be detected as a measuring current.

Electrochemical gas sensors with mediators are characterized by a low background current, high long-term stability and low cross sensitivity to interfering gases. Suitable mediators are so far only for specific detection reactions. The detection sensitivity of the electrochemical gas sensor is also influenced by its electrode material.

An electrochemical gas sensor with a plurality of electrodes and a measuring electrode of diamond-like carbon (DLC) is known from DE 199 39 011 C1 known. The measuring electrode is produced by means of a coating method in which diamond-like carbon is applied to a gas-permeable membrane by means of a sputtering process. The measuring electrodes made of DLC are very long-term stable.

An electrochemical gas sensor with a measuring electrode of boron- or nitrogen-doped diamond (BDD) goes out of the DE 101 44 862 A1 out. The measuring electrode material is applied as a thin layer to a porous, gas-permeable substrate. Such measuring electrodes are very long-term stable and have an extremely large potential window, so that even very difficult oxidizable substances can be implemented.

From the DE 10 2006 014 713 B3 An electrochemical measuring device is known in which the measuring electrode has carbon nanotubes. This sensor incorporates a transition-metal mediator that selectively repels SO2, eliminating the formation of elemental sulfur, but has low sensitivity to H2S.

DE 10 2006 014 714 B3 provides an electrochemical sensor for gases having a mediator compound based on transition metal salts. DE 10 2006 014 715 B3 provides an electrochemical gas sensor in which a mediator compound is both saturated in the electrolyte and as a bottom body in the electrolyte.

The invention has for its object to provide a mediator-based electrochemical gas sensor, which selectively detects hydrogen sulfide.

The object is achieved by the features of patent claim 1.

Advantageous embodiments of the invention will become apparent from the dependent claims.

The mediator system according to the invention is based on the fact that the hydrogen sulfide is oxidized to sulfuric acid and thus avoids the formation of elemental sulfur and at the same time a high sensitivity is achieved.

• – konstante Mediator-Konzentration bei variabler Luftfeuchte,
• – identische Gleichgewichtspotentiale an Mess- und Hilfselektrode, falls Mess- und Hilfselektrode aus Kohlenstoff bestehen,
• – Filterwirkung des Bodenkörpers und
• – Sensor kann unter anaeroben Bedingungen betrieben werden, falls die Mess- und Hilfselektrode aus Kohlenstoff bestehen und der Mediator deren Potential bestimmt.

Preferably, mediators are not completely soluble in an electrolyte solution. The use of suspensions or solutions of the mediator with bottom body offers a number of advantages, such as:

• - constant mediator concentration at variable humidity,
• - identical equilibrium potentials on the measuring and auxiliary electrodes, if the measuring and auxiliary electrodes are made of carbon,
• - Filter effect of the soil body and
• - Sensor can be operated under anaerobic conditions if the measuring and auxiliary electrodes are made of carbon and the mediator determines their potential.

The mediator compound of the invention utilizes transition metal metals. Suitable metalates here are vanadates, chromates, molybdate, tungstates, permanganates. Particularly advantageous are molybdates of a transition metal salt.

The conductive electrolyte used is preferably an aqueous lithium chloride solution, 2 molar-10 molar, preferably 3 molar, covering a wide range of temperature and humidity. When using organic solvents such as ethylene carbonate or propylene carbonate and ammonium halides can be used. It is also possible to use ionic liquids, for example substituted ammonium, phosphonium or imidazolium compounds.

The following describes the preparation of an aqueous electrolyte suspension.

• • Metallate: Chromate, Vanadate, Wolframate, bevorzugt jedoch Molybdate. Die Konzentration der Metallate liegt dabei zwischen 0,2 molar und 2 molar, vorzugsweise bei 1,0 molar.
• • Anorganische Säuren oder saure Salze wie NaHSO₄. Mit diesen Zusätzen konnten sowohl der Grundstrom als auch die t₉₀ Zeit deutlich reduziert werden.

To the aqueous lithium chloride solution is added so much copper chloride that the concentration is between 0.5 molar and 5.0 molar, preferably 3.0 molar. The following reagents are added to the resulting chloro complexes:

• • Metallates: Chromates, vanadates, tungstates, but preferably molybdate. The concentration of the metalates is between 0.2 molar and 2 molar, preferably at 1.0 molar.
• • Inorganic acids or acid salts such as NaHSO ₄ . With these additives, both the base current and the t ₉₀ time could be significantly reduced.

To stabilize the pH, polybasic carboxylic acids or their salts are suitable: preferably citric acid, phthalic acid or citrates and phthalates. Boric acid or its salts can also be used as polybasic acid.

The resulting concentration of the reagents should be 0.5 mol to 5.0 mol, preferably 1.0 mol per liter. In addition to the catalytic activity, these compounds also have pH buffering properties, allowing the sensors to be gassed for many hours without loss of sensitivity.

When assembling the individual components, a green solution forms, from which a precipitate precipitates after some time. As conducting electrolytes it is also possible to use hygroscopic alkali metal or alkaline earth metal halides, preferably chlorides, in aqueous solution. A particularly preferred formulation is 3.0 molar LiCl, 3.0 molar CuCl ₂ , 1.0 molar Li ₂ Mo O ₄ .

The measuring electrode is preferably made of diamond-like carbon (DLC). But it is also possible to use other carbon materials, such as carbon nanotubes or measuring electrodes of boron- or nitrogen-doped diamond (BDD) or noble metal thin-film electrodes.

Measuring electrodes made of carbon nanotubes (KNR) are long-term stable, easy to integrate into existing sensor designs, suitable for a large number of mediators and cost-effective to procure. There are only a few cross-sensitivities caused by the electrode material. This applies in particular to multi-walled carbon nanotubes (MW KNR). The carbon nanotubes are preferably used without binders.

Such measuring electrodes are wetted over the entire surface of the electrolyte solution, resulting in a large surface area for the electrochemical reaction. The measuring electrode according to the invention is preferably also gas-permeable. A measuring electrode made of KNR has a better conductivity than a comparable measuring electrode made of DLC.

The layer thickness of the carbon nanotubes on a measuring electrode is dependent on their structure. If the carbon nanotube tubes are in the form of multi-walled carbon nanotubes, the layer thickness is between one micrometer and one thousand micrometers, preferably between 50 micrometers and 150 micrometers. For single-walled carbon nanotubes, the layer thickness is between 0.5 microns and 100 microns, preferably between 10 microns and 50 microns.

The layer thickness is also dependent on the purity of the material. For a particularly pure material, the layer thickness tends to move at the lower end of the range.

Through the use of carbon nanotubes results in a large-area contact of the measuring electrode material with the analyte or with the converted mediator, so that a complete oxidation or reduction takes place. In this way it is prevented that a part of the analyte or the converted mediator diffuses into the electrolyte space.

If the measuring electrode is designed as a noble metal thin-film electrode, the layer thickness is below 600 micrometers. Thick-film electrodes have not been proven, since they have high base currents and low selectivities.

The auxiliary electrode is suitably made of a noble metal, for example gold, platinum or iridium, alternatively carbon nanotubes.

In addition to the auxiliary electrode, there may be a reference electrode or a protective electrode made of a noble metal or carbon nanotubes.

An embodiment of the invention is shown in the figure and explained below.

Show it:

1 an electrochemical gas sensor,

2 a fumigation curve,

3 a comparison of cross-sensitivities.

When in the 1 illustrated embodiment of the inventive electrochemical sensor 1 are in a sensor housing 2 a measuring electrode 3 diamond-like carbon (DLC) on a diffusion membrane 4 , a protective electrode 5 , a reference electrode 6 in a wick 7 and an auxiliary electrode 8th arranged. The interior of the sensor housing is with an electrolyte-mediator mixture 9 filled, the mediator additionally as a bottom body 10 is available. The electrodes 3 . 5 . 6 . 8th be using liquid-permeable nonwovens 11 . 12 . 13 . 14 kept at a fixed distance from each other. The gas enters through an opening 15 in the sensor housing 2 , The electrochemical sensor 1 is connected in a known manner to a potentiostat, not shown.

2 gives a typical fumigation curve 16 with the sensor according to the invention 1 again. The sensor 1 was exposed to a concentration of 1.96 ppm H ₂ S at a temperature of 20 ° Celsius and 50% relative humidity for 6 minutes. On the abscissa of 2 is the gassing time in seconds and plotted on the ordinate the sensor current I in microamps.

As averages of five sensors and five measurements result: I ₀ = 7 ± 2 nA (Background current) S = 3.0 ± 0.1 μA ppm -1 (Sensor signal in μA per ppm H ₂ S) D = 3.4 ± 1.6% (Drift) t _0-90 = 41.8 ± 18.6 s (Step response up to 90% of the maximum sensor signal)

The sensor according to the invention 1 is characterized by a very low background current I ₀ and by the large measuring range, since both concentrations of a few ppm can be measured as well as gas concentrations in the percentage range.

3 illustrates the cross-sensitivities of a conventional electrochemical gas sensor with noble metal thick film electrode over the sensor according to the invention 1 with the mediator connection. The bars with coarse hatching 17 refer to the conventional gas sensor and the bars with fine hatching 18 are the sensor according to the invention 1 assigned. On the abscissa of 3 are the examined gases and on the ordinate the sensor signal S is plotted in μA per ppm H ₂ S. Again 3 As can be seen, both sensors provide a roughly equal measurement signal when gassing with H ₂ S. However, the conventional gas sensor has significant cross sensitivities under the influence of moisture and in the gases NO, PH ₃ , A _s H ₃ , SO ₂ and B ₂ H ₆ . The sensor according to the invention 1 on the other hand has only one cross-sensitivity in SO ₂ . This cross-sensitivity, because it is only one gas, can be easily compensated by, for example, a second sensor measuring only the SO ₂ component.

LIST OF REFERENCE NUMBERS

1

electrochemical sensor

2

sensor housing

3

measuring electrode

4

diffusion membrane

5

guard electrode

6

reference electrode

7

wick

8th

auxiliary electrode

9

Electrolyte mediator mixture

10

sediment

11, 12, 13, 14

fleece

15

opening

16

Begasungskurve

17

rough hatching

18

fine hatching

Claims (14)

Electrochemical gas sensor for the detection of H ₂ S in a gas sample with at least one measuring electrode ( 3 ) and another electrode ( 8th ) in an electrolyte solution ( 9 ), the electrolyte solution ( 9 ) contains a mediator compound in the form of transition metal metals, with a concentration of the metallates in a range between 0.2 molar and 2.0 molar, and ionic liquids in the form of substituted ammonium, phosphonium or imidazolium compounds. Electrochemical gas sensor according to claim 1, characterized in that the measuring electrode ( 3 ) consists of diamond-like carbon (DLC), boron or nitrogen-doped diamond (BDD) or carbon nanotubes. Electrochemical gas sensor according to claim 1, characterized in that the measuring electrode ( 3 ) is designed as a noble metal thin-film electrode with a layer thickness below 600 microns. Electrochemical gas sensor according to one of claims 1 to 3, characterized in that are used as metalates vanadates, chromates, tungstates, permanganates, preferably Molybdate with a transition metal salt. Electrochemical gas sensor according to claim 4, characterized in that the transition metal salt is a copper salt, preferably copper chloride. An electrochemical gas sensor according to claim 5, characterized in that the concentration of the transition metal salt is in a range between 0.5 molar and 5.0 molar, preferably at 3.0 molar. Electrochemical gas sensor according to one of claims 1 to 6, characterized in that polybasic acids or their salts are added. Electrochemical gas sensor according to claim 7, characterized in that as polybasic acid citric acid, phthalic acid or citrates and phthalates are added. Electrochemical gas sensor according to claim 7, characterized in that are used as the polybasic acid boric acid or its alkali metal salts. Electrochemical gas sensor according to one of claims 1 to 6, characterized in that inorganic acids or acid salts, such as NaHSO ₄ , are added. Electrochemical gas sensor according to one of the preceding claims, characterized in that the electrolyte solution ( 9 ) as the conductive salt hygroscopic alkali or alkaline earth metal salts, preferably lithium chloride. Electrochemical gas sensor according to claim 11, characterized in that a lithium chloride solution in the range 2.0 molar-10 molar, preferably 3.0 molar, is used. Electrochemical gas sensor according to one of the preceding claims, characterized in that the electrolyte solution ( 9 ) contains as solvent water. Electrochemical gas sensor according to one of the preceding claims, characterized in that the electrolyte solution ( 9 ) organic solvent sulfolane, ethylene carbonate or propylene carbonate.

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